The invention relates to a steering booster device, having a torque-sensor shaft being provided between a first shaft carrying the steering wheel and a second shaft connected to the gear of the steering booster.
Electric steering boosters are increasingly being used in motor vehicles. These comprise an actuating device (electric motor), an electronic control unit and a torque sensor. The torque sensor is used to measure the torque that the driver exerts on the steering shaft by means of the steering wheel. The torque-dependent deformation of the torque-sensor shaft is measured by a suitable sensor and converted into a signal, which is evaluated by the electronic control unit, the latter controlling the electric motor accordingly. A torque-sensor shaft of this kind is described in EP 0 570 066 A1, for example. The following problems can arise when integrating the torque sensor into the steering shaft.
The additional support for the steering shaft at the ends of the torque-sensor shaft can lead to stresses and unwanted bending moments given the ever-present tolerances and concentricity errors.
Another problem can arise if the torque sensor is protected by a mechanical overload stop since high accuracy of manufacture is required for the overload stop in the case of highly rigid torque sensors and even small angular errors lead to large variations in the stop torque.
Another disadvantageous effect is that vibrations are transmitted directly to the steering wheel from the engine compartment by the steering shaft.
The object on which the invention is based is therefore to eliminate these problems by simple and inexpensive means.
According to the invention, the objective is achieved by virtue of the fact that at least one end of the torque-sensor shaft is supported flexibly in the first or second shaft by damping elements. This makes it possible to reduce or avoid stresses and the occurrence of bending moments in the steering shaft.
An advantageous refinement of the steering booster device according to the invention comprises providing support by means of rubber elements. This material is well suited for flexible support and can be obtained at a reasonable cost. At the same time, damping elements made of caoutchouc and plastics, especially elastomers, are suitable.
The rubber elements furthermore isolate the steering wheel from vibrations from the engine compartment since the direct metallic connection is interrupted and the flexible connection via the rubber elements filters and reduces vibrations. This improves the level of comfort for the driver.
Another refinement of the steering booster device according to the invention envisages that both ends are supported flexibly by damping elements. This enhances the effect of avoiding and reducing vibrations on the steering wheel and further reduces the occurrence of stresses and moments in the steering column. Moreover, two flexible rubber elements at both ends of the stiff sensor shaft form a double Cardan joint that can be produced economically in large numbers.
The invention increases the permissible tolerance for the manufacture of the overload clutch since stiffness is reduced and the angle of rotation for a given torque is increased. As a result, the overload safeguard can be manufactured more economically since the angular error does not cause such a large change in the stop torque. The rubber elements are furthermore likewise protected from overloading.
In the device according to the invention, an advantageous overload stop is formed by virtue of the fact that the first and the second shaft are designed as hollow shafts in the region of the torque-sensor shaft, that an annular rubber element is arranged at the base of each cavity, and that the facing ends of the hollow shafts are designed as a dog clutch with a given play for overload prevention.
In another advantageous refinement of the invention, it is envisaged that the stiffness of the damping elements is such that the characteristic of the control loop incorporating the steering booster, the shafts and the steering wheel is tuned in an optimum manner, in particular has no point of resonance.